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energies
Article
Experimental Investigations of Physical and ChemicalProperties for Microalgae HTL Bio-Crude Using aLarge Batch Reactor
Farhad M. Hossain 1,2,*, Jana Kosinkova 1,2, Richard J. Brown 1,2, Zoran Ristovski 1,2,Ben Hankamer 3, Evan Stephens 3 and Thomas J. Rainey 1,2,*
1 Biofuel Engine Research Facility, Queensland University of Technology (QUT), Brisbane, QLD 4001,Australia; [email protected] (J.K.); [email protected] (R.J.B.);[email protected] (Z.R.)
2 International Laboratory for Air Quality and Health, Queensland University of Technology (QUT), Brisbane,QLD 4001, Australia
3 The Institute for Molecular Bioscience, University of Queensland, 306 Carmody Road, St. Lucia, QLD 4072,Australia; [email protected] (B.H.); [email protected] (E.S.)
* Correspondence: [email protected] (F.M.H.); [email protected] (T.J.R.); Tel.: +61-3138-1977 (T.J.R.)
Academic Editor: Thomas E. AmidonReceived: 21 December 2016; Accepted: 28 March 2017; Published: 5 April 2017
Abstract: As a biofuel feedstock, microalgae has good scalability and potential to supply a significantproportion of world energy compared to most types of biofuel feedstock. Hydrothermal liquefaction(HTL) is well-suited to wet biomass (such as microalgae) as it greatly reduces the energy requirementsassociated with dewatering and drying. This article presents experimental analyses of chemicaland physical properties of bio-crude oil produced via HTL using a high growth-rate microalgaScenedesmus sp. in a large batch reactor. The overarching goal was to investigate the suitability ofmicroalgae HTL bio-crude produced in a large batch reactor for direct application in marine dieselengines. To this end we characterized the chemical and physical properties of the bio-crudes produced.HTL literature mostly reports work using very small batch reactors which are preferred by researchers,so there are few experimental and parametric measurements for bio-crude physical properties, such asviscosity and density. In the course of this study, a difference between traditionally calculated valuesand measured values was noted. In the parametric study, the bio-crude viscosity was significantlycloser to regular diesel and biodiesel standards than transesterified (FAME) microalgae biodiesel.Under optimised conditions, HTL bio-crude’s high density (0.97–1.04 kg·L−1) and its high viscosity(70.77–73.89 mm2·s−1) had enough similarity to marine heavy fuels. although the measured higherheating value, HHV, was lower (29.8 MJ·kg−1). The reaction temperature was explored in the range280–350 ◦C and bio-crude oil yield and HHV reached their maxima at the highest temperature. Slurryconcentration was explored between 15% and 30% at this temperature and the best HHV, O:C, andN:C were found to occur at 25%. Two solvents (dichloromethane and n-hexane) were used to recoverthe bio-crude oil, affecting the yield and chemical composition of the bio-crude.
Keywords: microalgae; hydrothermal liquefaction (HTL); bio-crude; fatty acid methyl esters (FAME);fuel properties
1. Introduction
Microalgae are of considerable interest for the production of next-generation biofuels [1,2] that areindistinguishable from petroleum fuels based on their properties [3]. This is because using microalgaefor biofuel production would have fewer adverse effects on food supply and other agriculture [4,5] as
Energies 2017, 10, 467; doi:10.3390/en10040467 www.mdpi.com/journal/energies
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they can be cultivated on non-arable land using fresh, waste, or saline water sources and enable moreefficient nutrient recycling and achieve higher productivities [6,7].
A range of conversion techniques are under development to generate biofuels from microalgae.These include solvent extraction followed by trans-esterification to produce fatty acid methyl esters(FAME), fermentation to alcohols, as well as thermochemical conversion pathways, such as pyrolysisand hydrothermal liquefaction (HTL) [8,9], to produce bio-crude oils. HTL efficiently convertswet microalgae biomass feedstock into bio-crude oil [10–12] as it eliminates the need for expensivepre-drying of the raw material. Compared to trans-esterifying lipids obtained from microalgae bysolvent extraction, HTL has the potential to require less energy for the conversion process which wouldimprove production costs [13]. The high solvent losses associated with FAME production means itsfinancial viability is directly related to the lipid content (high lipid content being better) whereas HTLcan make use of more highly productive microalgae species (i.e., more tonnes per hectare) which havehigher carbohydrate content, lower lipid content and the potential for additional co-products [14].These advantages of HTL make it a competitive route for the conversion of raw microalgae biomass tofuels [10–12].
HTL converts microalgae biomass into bio-crude oil, as well as aqueous, gas, and solid phaseproducts at elevated pressures (5–24 MPa) and moderate temperatures (250–400 ◦C) [15,16]. A widerange of microalgae biomass feedstocks has been explored using HTL, including laboratory andcommercially grown strains Botryococcus braunii [17], Arthrospira (Spirulina), Scenedesmus sp. [18],and Tetraselmis sp. [2]. Among the green microalgae the most common Scenedesmus sp. has highproductivity although the lipid yield can be optimised to reach over 60% [19], which makes thisstrain attractive for biofuel production. In general, the variation in the biochemical composition,particularly the carbon chain length and the degree of saturation, affects the conversion rate and thechemical-physical properties of the HTL bio-crude [20,21].
The effect of operating conditions such as residence time, temperature, slurry concentration, andcatalyst, on physical and chemical properties of microalgae HTL bio-crude have been investigatedpreviously in limited studies [22–25]. In this study, three temperatures and slurry concentrations wereinvestigated in the range 280–350 ◦C and 15–30%, respectively; temperature and concentration, as wellas reaction time of 60 min was based loosely on the literature [10,23]. Higher temperatures were notexplored so as to remain below water’s supercritical point. Jena et al. [23] demonstrated that amongthe various process parameters the temperature had the strongest influence on the higher heatingvalue (HHV), viscosity and chemical composition. HTL produces bio-crude oil that, by weight, canhave 6–8 times higher oxygen and nitrogen content than heavy fuel oil (HFO) [26]. The inorganic saltsin many types of HTL bio-crude oil require modifications to be compatible with a traditional refiningprocess [2,10,27,28]. Bio-crude oil may also need subsequent upgrading to improve quality and reducethese undesired components.
Most experimental research in HTL continues to use small batch reactors (less than 1000 mL),while scale-up reactors are usually continuous. The kinetics of small size reactors or autoclaves maydiffer from large ones, both batch and continuous, which lead to different chemical reactions. Therefore,the results from these reactors are not fully reliable in terms of scale-up. Batch reactors are commonlyused to validate the process conditions for further commercial scale. Although industry tends to prefercontinuous reactors to batch reactors because of higher energy efficiency, the steady production rateand uniformity of product, large batch reactors still have several key advantages [29]:
• Pumping the biomass slurry into a continuous reactor at high pressure and temperature remainsa technological challenge at the industrial scale;
• Ability to easily switch between feedstocks; and• For a factory, individual reactors can be taken out-of-service for maintenance.
Due to the lack of bio-crude for testing, most groups only reported values at optimised parameters.The aim of this study was to conduct more comprehensive experimental analyses with three key
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objectives: (i) to empirically investigate the relationship between the reaction parameters andexperimentally determined values physical properties of bio-crude; (ii) to evaluate how the chemicalcomposition varies with operating conditions; and (iii) to identify the best possible fuel physical andchemical properties of microalgae HTL bio-crude. Both physical and chemical properties are neededto meet legislated diesel and FAME biodiesel standards. In further analysis, the physical properties ofthe best bio-crude were compared with other fuel standards to determine the similarities and revealareas for improvement.
2. Materials and Methods
2.1. Hydrothermal Liquefaction (HTL)
The experiments were performed in a 1.8 L batch reactor system (Parr Instruments Co., Moline,IL, USA) and data were collected across three physical variables: temperature, slurry concentration,and solvents for recovering the bio-crude from the liquid mixture. The experimental temperaturesused were 280 ◦C, 300 ◦C, and 350 ◦C, and solid concentrations in the slurry were 15%, 25%, and30% by weight. When loading the reactor, the headspace was purged thoroughly using nitrogento remove oxygen, and pre-pressurised to 2 bar with nitrogen gas. The reactor was heated to thedesired temperature (heating rate ~3.3 ◦C/min) and held constant for one hour which is consistentwith the literature [23,30–34]. At the end of the reaction time, the reactor was cooled by passing waterthrough an internal cooling tube until room temperature was reached. Experiments were performed induplicate and the average yield of two runs is reported.
After the gas was vented from the reactor, the vessel was opened and the mixture was separated(following the steps in Figure 1). The walls of the reactor were washed thoroughly with solvent(dichloromethane or n-hexane) and mixed with the liquid phase. The amount of solvent addedwas determined as the volume of the solvent per mass dry weight of algae. The solid phase wasremoved from the liquid mixture by vacuum filtration before being washed with the remaining solvent.The solids were then oven-dried at 105 ◦C overnight.
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chemical composition varies with operating conditions; and (iii) to identify the best possible fuel physical and chemical properties of microalgae HTL bio-crude. Both physical and chemical properties are needed to meet legislated diesel and FAME biodiesel standards. In further analysis, the physical properties of the best bio-crude were compared with other fuel standards to determine the similarities and reveal areas for improvement.
2. Materials and Methods
2.1. Hydrothermal Liquefaction (HTL)
The experiments were performed in a 1.8 L batch reactor system (Parr Instruments Co., Moline, IL, USA) and data were collected across three physical variables: temperature, slurry concentration, and solvents for recovering the bio-crude from the liquid mixture. The experimental temperatures used were 280 °C, 300 °C, and 350 °C, and solid concentrations in the slurry were 15%, 25%, and 30% by weight. When loading the reactor, the headspace was purged thoroughly using nitrogen to remove oxygen, and pre-pressurised to 2 bar with nitrogen gas. The reactor was heated to the desired temperature (heating rate ~3.3 °C/min) and held constant for one hour which is consistent with the literature [23,30–34]. At the end of the reaction time, the reactor was cooled by passing water through an internal cooling tube until room temperature was reached. Experiments were performed in duplicate and the average yield of two runs is reported.
After the gas was vented from the reactor, the vessel was opened and the mixture was separated (following the steps in Figure 1). The walls of the reactor were washed thoroughly with solvent (dichloromethane or n-hexane) and mixed with the liquid phase. The amount of solvent added was determined as the volume of the solvent per mass dry weight of algae. The solid phase was removed from the liquid mixture by vacuum filtration before being washed with the remaining solvent. The solids were then oven-dried at 105 °C overnight.
Figure 1. Hydrothermal liquefaction (HTL) product recovery workflow.
Figure 1. Hydrothermal liquefaction (HTL) product recovery workflow.
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The liquid phase mixture was poured into a separation funnel and the water-insoluble componentsseparated (organic phase) were separated from the water soluble (aqueous) phase. The solventwas evaporated from the bio-crude by a rotary vacuum evaporator. Due to the volatile nature ofdichloromethane (DCM) we have used 40 ◦C (760 Tor) and placed an additional trap between thevacuum source and the condenser unit, whereas for n-hexane the temperature was set at 68 ◦C (760 Tor).The evaporation was continued until no further yield was obtained. The time for the DCM was shorterthan that for n-hexane.
2.2. Raw Materials
A robust and fast growing green freshwater microalga Scenedesmus sp. which had been isolatedas part of previous work [35] was used for these experiments. The Scenedesmus sp. bulk biomasswas produced at the University of Queensland’s Solar Biofuels Research Centre facility at PinjarraHills near Brisbane, Australia. High density slurry was frozen and stored at −20 ◦C, directly afterharvest to prevent degradation during the course of the HTL experiments. Proximate analyses wereperformed according to American Society for Testing and Materials (ASTM) standards and usinga thermo-gravimetric analyzer (TGA–NETZSCH thermal analyser, NETZSCH Australia Pty Ltd.,New South Wales, Australia). Ultimate analyses were carried out using a LECO TruSpec Micro CHNSelemental analyser (LECO Corporation, Saint Joseph, MI, USA) (Table 1). The HHV of the driedmicroalgae sample was calculated according to the formula used by Demirbas [36] and Friedl [37],which were 18.07 MJ·kg−1 and 19.5 MJ·kg−1, respectively, for Scenedesmus species microalgae. The lipidcontent for the samples used in this study was typically 15–20%.
Table 1. Microalgae proximate and ultimate analyses data.
Proximate Analyses Ultimate Analyses
Composition Percentage of Weight Element Percentage of Weight
Fixed carbon 24.8 C 46.3Volatile matter 67.3 H 6.9
Ash 3.2 O 32.3Moisture 85 N 7.3
- - S 2.3
2.3. Analytical Methods
The conversion and product yields were defined as the mass fraction of the respective product(i.e., bio-crude and solid residue) as a function of the initial mass of biomass. For approximation,the total yield of gas + aqueous products were determined by difference according to the approachcommonly used in the literature (e.g., [38]).
Solid residues (wt.%) =
(mass of solid residuesmass of raw material
)× 100% (1)
Bio − crude yield (wt.%) =
(mass of bio − crude
mass of raw material
)× 100% (2)
2.4. Bio-Crude Properties Measurements
The chemical and physical bio-crude properties such as HHV, viscosity, density, and chemicalcomposition were measured experimentally. HHV of the bio-crude was measured with a Parr 6200compensated jacket calorimeter (Moline, IL, USA). For comparative purposes, the correlations whichare used for fossil fuels (liquid, gas and coal) form the basis of calculation applied in other researcharticles. Boie’s and Dulong’s formulae are shown Equations (3) and (4), respectively [36,39].
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HHV (MJ/kg) = 35.16 C + 116.225 H − 10.09 O + 6.28 N + 10.465 S (3)
HHV (MJ/kg) = 0.3383 C + 1.422 (H − O/8) (4)
A Brookfield DV-III ultra-programmable rheometer (Brookfield engineering laboratories, Inc.,Middleboro, MA, USA) was used to measure the viscosity of the bio-crude at constant temperature.The accuracy of the rheometer for viscosity measurements is ±1.0% of full scale range for a specificspindle running at a specific speed. The density of the bio-crude was measured using Germanindustrial standard (DIN) 1306.
Gas chromatography with mass spectroscopy (GC-MS) was used to identify the chemicalcompositions in the bio-crude samples. GC-MS analyses were performed using a Thermoscientific,Trace 1310 system (Thermo Scientific™, Milan, Italy), equipped with a single quadrupole mass selectivedetector (ISQ). Each sample was dissolved and diluted in dichloromethane. The injector was set to250 ◦C and a Thermo TG-5MS (30 m long, 0.25 mm ID, 0.25 mm film) column was used. The ovenwas programmed at an initial temperature at 50 ◦C (held 1/min) then heated at a constant rate of10 ◦C·min−1 until a temperature of 250 ◦C was reached, and then held for 9 min with a split ratio of1:25 and a column flow of 1.4 mL·min−1. The MS detector scanned from 40 to 400 m/z with a solventcut time of 1.8 min and the ion source and transfer line temps were both set at 250 ◦C. The carriermode was set to constant flow.
3. Results and Discussion
To investigate the influence of reaction conditions on product yields, HTL was conducted using arange of reaction temperatures, solid concentrations, and two types of solvents (dichloromethane andn-hexane) to recover the bio-crude.
3.1. Influence of Solvents in Product Separation
After completion of HTL, the bio-crude must be separated from the aqueous and solid phases.The most common solvents reported in the literature, with a focus on maximising bio-crude yield arethe organic solvents dichloromethane, chloroform, acetone, and n-hexane [34,40]. However, there wasinsufficient information about the effect of these polar and non-polar solvents on the bio-crude quality.The data presented in Figure 2 was obtained from experimental runs with 25% slurry concentration at350 ◦C for 60 min in a nitrogen atmosphere.
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A Brookfield DV-III ultra-programmable rheometer (Brookfield engineering laboratories, Inc., Middleboro, MA, USA) was used to measure the viscosity of the bio-crude at constant temperature. The accuracy of the rheometer for viscosity measurements is ±1.0% of full scale range for a specific spindle running at a specific speed. The density of the bio-crude was measured using German industrial standard (DIN) 1306.
Gas chromatography with mass spectroscopy (GC-MS) was used to identify the chemical compositions in the bio-crude samples. GC-MS analyses were performed using a Thermoscientific, Trace 1310 system (Thermo Scientific™, Milan, Italy), equipped with a single quadrupole mass selective detector (ISQ). Each sample was dissolved and diluted in dichloromethane. The injector was set to 250 °C and a Thermo TG-5MS (30 m long, 0.25 mm ID, 0.25 mm film) column was used. The oven was programmed at an initial temperature at 50 °C (held 1/min) then heated at a constant rate of 10 °C·min−1 until a temperature of 250 °C was reached, and then held for 9 min with a split ratio of 1:25 and a column flow of 1.4 mL·min−1. The MS detector scanned from 40 to 400 m/z with a solvent cut time of 1.8 min and the ion source and transfer line temps were both set at 250 °C. The carrier mode was set to constant flow.
3. Results and Discussion
To investigate the influence of reaction conditions on product yields, HTL was conducted using a range of reaction temperatures, solid concentrations, and two types of solvents (dichloromethane and n-hexane) to recover the bio-crude.
3.1. Influence of Solvents in Product Separation
After completion of HTL, the bio-crude must be separated from the aqueous and solid phases. The most common solvents reported in the literature, with a focus on maximising bio-crude yield are the organic solvents dichloromethane, chloroform, acetone, and n-hexane [34,40]. However, there was insufficient information about the effect of these polar and non-polar solvents on the bio-crude quality. The data presented in Figure 2 was obtained from experimental runs with 25% slurry concentration at 350 °C for 60 min in a nitrogen atmosphere.
Figure 2. Effect of solvent on the recovery of bio-crude, solids and gas + aqueous components after HTL treatment (350 °C, 60 min, 25% slurry concentration).
Suitable nonpolar (n-hexane) and more polar (dichloromethane; DCM) solvents were selected based on previous studies and the product yields and chemical composition were compared (see Section 3.4). For the same algae species, and under identical reaction conditions, bio-crude yield
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Yiel
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Figure 2. Effect of solvent on the recovery of bio-crude, solids and gas + aqueous components afterHTL treatment (350 ◦C, 60 min, 25% slurry concentration).
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Suitable nonpolar (n-hexane) and more polar (dichloromethane; DCM) solvents were selectedbased on previous studies and the product yields and chemical composition were compared (seeSection 3.4). For the same algae species, and under identical reaction conditions, bio-crude yieldvaried from 31.2% (wt) with n-hexane to 33.6% (wt) with DCM. This differed from the results ofValdez et al. [34] who obtained the highest bio-crude yield (39%) with nonpolar solvents (hexadecaneand decane) and the lowest value with 30% DCM for Nannochloropsis sp. at 350 ◦C for 60 min. Thissupports the view that both biomass properties and the solvent type used for separating products afterHTL has an influence on bio-crude recovery. However, as n-hexane has a higher boiling point thanDCM, this could lead to higher losses of volatile compounds during the evaporation process and thiscould possibly account for the small difference (2.4%). Despite this, DCM was used as a solvent ofchoice for product separation in all subsequent experiments.
3.2. Effect of Reaction Temperature on Yield and HHV
The effect of three different temperatures (280 ◦C, 300 ◦C, 350 ◦C) was investigated using a 60 minreaction time and a nitrogen atmosphere at an initial pressure of 2 bar (Figure 3). Bio-crude yieldincreased with temperature from 24.5% at 280 ◦C to 32.5% at 300 ◦C, and reached a maximum of 33.6%at 350 ◦C, in the range analysed. Similar results for temperature’s influence on bio-crude yield havebeen reported recently [23,32,41]. The HHV was characterized for bio-crude generated at temperaturesof 300 ◦C and 350 ◦C only, due to an insufficient amount of bio-crude at 280 ◦C. The HHV followed thetrend of bio-crude yield; it increased from 26.1 MJ·kg−1 to 29.8 MJ·kg−1, which was approximately50% higher than in the original microalgae biomass. The solid residue yield decreased gradually from20.1% to 15.3% with rising temperature suggesting organic conversion mostly into the aqueous and gasphases. These outcomes generally support processing at the higher temperature range where bio-crudeyield was maximized. Higher temperatures than these were not investigated because of potentialcorrosion issues related to supercritical fluids. To this end, 350 ◦C was used for future experiments.
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varied from 31.2% (wt) with n-hexane to 33.6% (wt) with DCM. This differed from the results of Valdez et al. [34] who obtained the highest bio-crude yield (39%) with nonpolar solvents (hexadecane and decane) and the lowest value with 30% DCM for Nannochloropsis sp. at 350 °C for 60 min. This supports the view that both biomass properties and the solvent type used for separating products after HTL has an influence on bio-crude recovery. However, as n-hexane has a higher boiling point than DCM, this could lead to higher losses of volatile compounds during the evaporation process and this could possibly account for the small difference (2.4%). Despite this, DCM was used as a solvent of choice for product separation in all subsequent experiments.
3.2. Effect of Reaction Temperature on Yield and HHV
The effect of three different temperatures (280 °C, 300 °C, 350 °C) was investigated using a 60 min reaction time and a nitrogen atmosphere at an initial pressure of 2 bar (Figure 3). Bio-crude yield increased with temperature from 24.5% at 280 °C to 32.5% at 300 °C, and reached a maximum of 33.6% at 350 °C, in the range analysed. Similar results for temperature’s influence on bio-crude yield have been reported recently [23,32,41]. The HHV was characterized for bio-crude generated at temperatures of 300 °C and 350 °C only, due to an insufficient amount of bio-crude at 280 °C. The HHV followed the trend of bio-crude yield; it increased from 26.1 MJ·kg−1 to 29.8 MJ·kg−1, which was approximately 50% higher than in the original microalgae biomass. The solid residue yield decreased gradually from 20.1% to 15.3% with rising temperature suggesting organic conversion mostly into the aqueous and gas phases. These outcomes generally support processing at the higher temperature range where bio-crude yield was maximized. Higher temperatures than these were not investigated because of potential corrosion issues related to supercritical fluids. To this end, 350 °C was used for future experiments.
Figure 3. Influence of the reaction temperature on products yields at 25% solids concentration and 60 min reaction time in nitrogen atmosphere (2 bar at commencement). Values are for yield data.
3.3. Effects of Solid Concentration on Yield and HHV
Liquefaction of Scenedesmus sp. was conducted at 350 °C for 60 min in a nitrogen atmosphere using a range of algae slurry concentrations between 15% and 30% by weight (Figure 4). These results show that the solid concentration of algae slurry may have had a minor effect upon the bio-crude yield (ranging between ~28.9% and 33.6%) and HHV (26.5–29.8 MJ·kg−1) with the highest yields and HHV observed at 25% biomass concentration. However, the slurry concentration did have a significant effect upon chemical speciation (see Section 3.4). A similar trend for bio-crude and the water soluble fraction was reported by Jena et al. [23]. The fact that bio-crude yield peaked at the 25% slurry concentration suggests that the substrate was limiting at lower concentrations and that H+ and
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280 300 350
HHV
(MJ/
kg)
Yiel
d (%
wt.)
Temperature (°C)
Bio-crude SR Gas + aqueous HHV
Figure 3. Influence of the reaction temperature on products yields at 25% solids concentration and60 min reaction time in nitrogen atmosphere (2 bar at commencement). Values are for yield data.
3.3. Effects of Solid Concentration on Yield and HHV
Liquefaction of Scenedesmus sp. was conducted at 350 ◦C for 60 min in a nitrogen atmosphereusing a range of algae slurry concentrations between 15% and 30% by weight (Figure 4). These resultsshow that the solid concentration of algae slurry may have had a minor effect upon the bio-crude yield(ranging between ~28.9% and 33.6%) and HHV (26.5–29.8 MJ·kg−1) with the highest yields and HHVobserved at 25% biomass concentration. However, the slurry concentration did have a significant effect
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upon chemical speciation (see Section 3.4). A similar trend for bio-crude and the water soluble fractionwas reported by Jena et al. [23]. The fact that bio-crude yield peaked at the 25% slurry concentrationsuggests that the substrate was limiting at lower concentrations and that H+ and OH− ions responsiblefor liquefaction become limiting at higher concentrations [23]. The presence of active hydrogenformed from the water under HTL conditions stabilizes the biomass liquefaction intermediates. Thisprevents formation of more new compounds that do not decompose easily, producing a higher yieldof bio-crude [42]. The yield of the gas + aqueous fraction ranged between 51.1% and 60.6% overthe slurry solid concentration range tested and the lowest yield 51.1% was observed at 25% solidconcentration which corresponded with the highest bio-crude yields. Solid residues increased from10.5% to 17.1% with slurry concentration due to the increase of solid mass fraction in the slurry. Thiscorresponded with the report of Jena et al. [23] where solid residues had a small increase (5.4–7%) withsolids concentration in range from 10% to 50% [23].
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OH− ions responsible for liquefaction become limiting at higher concentrations [23]. The presence of active hydrogen formed from the water under HTL conditions stabilizes the biomass liquefaction intermediates. This prevents formation of more new compounds that do not decompose easily, producing a higher yield of bio-crude [42]. The yield of the gas + aqueous fraction ranged between 51.1% and 60.6% over the slurry solid concentration range tested and the lowest yield 51.1% was observed at 25% solid concentration which corresponded with the highest bio-crude yields. Solid residues increased from 10.5% to 17.1% with slurry concentration due to the increase of solid mass fraction in the slurry. This corresponded with the report of Jena et al. [23] where solid residues had a small increase (5.4–7%) with solids concentration in range from 10% to 50% [23].
Figure 4. Effect of slurry concentration on the recovery of bio-crude, solids and gas + aqueous components after HTL treatment (350 °C, 60 min). Standard deviations are based on two replicates; values provided are for yield data.
3.4. Chemical Characterisation of Bio-Crude Oil
The chemical composition of each bio-crude oil sample was characterized by GC-MS. Due to the complex composition of the bio-crude oil only abundant compounds were evaluated based on the peak areas (defined by the percentage of the chromatographic area of the compound out if the total area). GC-MS revealed distinct amounts of chemical compounds which consisted of more than 2% of the total area within the retention time range of 3–34 min. Around 64 compounds were identified which accounted for approximately 80–90% of the total peak area from all bio-oil samples and 28 of them are listed in Tables 2 and 3. The majority of the compounds obtained were cyclic nitrogenates (e.g., pyrolle, pyrazine, piperidine) and cyclic and aromatic oxygenates (e.g., phenols, ketones) which are similar to previous studies [18,23,31]. These compounds are formed from carbohydrates and proteins obtained from the feedstock which undergo depolymerisations, decompositions, and reformation [43]. The minor compounds were mostly hydrocarbons and esters which may be derived from lipid content. Lipids can produce stable hydrocarbons via decarboxylation and decarbonylation reactions [43,44]. The tables show a comparison of the identified chemical compounds in the bio-crude oil under different reaction conditions.
Table 2 presents only the key compounds that were identified through GC-MS for both solvents. It shows the effectiveness of the n-hexane and dichloromethane in terms of the extraction of bio-crude oil from the water soluble fraction, independently. Dichloromethane and n-hexane extracted 57.1% and 40.2% of heterocyclic and aromatic compounds (mostly nitrogenated) from the water, respectively. Ketones were the most abundant oxygenated compounds. Neither solvent had a strong influence on aliphatic recovery rates, although a slight increase was observed with dichloromethane. Valdez et al. [34] also reported that the total yields of aliphatic compounds did not vary significantly between the polar to nonpolar solvents. They also found that dichloromethane extracted more light
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(MJ/
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Figure 4. Effect of slurry concentration on the recovery of bio-crude, solids and gas + aqueouscomponents after HTL treatment (350 ◦C, 60 min). Standard deviations are based on two replicates;values provided are for yield data.
3.4. Chemical Characterisation of Bio-Crude Oil
The chemical composition of each bio-crude oil sample was characterized by GC-MS. Due to thecomplex composition of the bio-crude oil only abundant compounds were evaluated based on the peakareas (defined by the percentage of the chromatographic area of the compound out if the total area).GC-MS revealed distinct amounts of chemical compounds which consisted of more than 2% of thetotal area within the retention time range of 3–34 min. Around 64 compounds were identified whichaccounted for approximately 80–90% of the total peak area from all bio-oil samples and 28 of themare listed in Tables 2 and 3. The majority of the compounds obtained were cyclic nitrogenates (e.g.,pyrolle, pyrazine, piperidine) and cyclic and aromatic oxygenates (e.g., phenols, ketones) which aresimilar to previous studies [18,23,31]. These compounds are formed from carbohydrates and proteinsobtained from the feedstock which undergo depolymerisations, decompositions, and reformation [43].The minor compounds were mostly hydrocarbons and esters which may be derived from lipid content.Lipids can produce stable hydrocarbons via decarboxylation and decarbonylation reactions [43,44].The tables show a comparison of the identified chemical compounds in the bio-crude oil under differentreaction conditions.
Table 2 presents only the key compounds that were identified through GC-MS for both solvents.It shows the effectiveness of the n-hexane and dichloromethane in terms of the extraction ofbio-crude oil from the water soluble fraction, independently. Dichloromethane and n-hexane extracted57.1% and 40.2% of heterocyclic and aromatic compounds (mostly nitrogenated) from the water,
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respectively. Ketones were the most abundant oxygenated compounds. Neither solvent had a stronginfluence on aliphatic recovery rates, although a slight increase was observed with dichloromethane.Valdez et al. [34] also reported that the total yields of aliphatic compounds did not vary significantlybetween the polar to nonpolar solvents. They also found that dichloromethane extracted more lightlow molecular products such as aromatics, nitrogen-, oxygen-, and sulphur-obtaining compounds,than n-hexane [34].
Table 2. Major compounds of recovered bio-oils obtained from HTL (350 ◦C, 60 min, 2 bar nitrogenatmosphere) using two different extraction solvents (DCM and n-hexane).
RT (Retention Times, min) Name of CompoundArea %
DCM n-Hexane
4.5 4-hydroxy-4-methyl-2-pentanone 5.03 -4.82 Ethylbenzene 8.86 3.384.98 1,4-dimethyl benzene 11.99 6.345.44 1,3-dimethyl benzene 3.44 5.125.82 2,5-dimethyl pyrazine 4.09 -6.91 2,6-dimethyl-4-heptanone - 4.747.54 Trimethyl pyrazine 2.76 4.37.79 2,3,5-trimethyl-1H-pyrrole - 3.068.18 2,3-dimethyl-2-cyclopenten-1-one 5.32 4.368.49 3,7-dimethyl undecane - 2.828.77 4-methyl phenol 3.53 4.459.06 1-acetate-1,2,3-propanetriol 2.99 7.119.21 Undecane - 3.919.32 1-ethyl-2-pyrrolidinone 10.38 -
10.08 2,2,5,5-tetramethyl-3-cyclopenten-1-one - 5.1511.42 1-butyl-2-pyrrolidinone 4.25 2.7212.93 1-pentyl piperidine 3.78 4.8413.49 2-methyl-3-hydroxy-2,4,4-trimethylpentyl ester, propanoic acid - 5.1124.91 Di(2-propylpentyl) ester, phthalic acid 10.78 -Total - 77.2 67.41
-, chemical compounds either not detected or peak area less than 2%.
Table 3 presents the variation of key chemical compounds from the bio-crude oil samplesproduced with a microalgae mass fraction in the feed ranging from 15–30% under various temperatures(280–350 ◦C). The main compounds include: nitrogenated compounds (pyrolle, pyrazine, piperidine);oxygenated compounds (phenols, ketones, esters); aliphatics (alkanes, alkenes); and aromatics (benzene).
Table 3. Major compounds of recovered bio-oils obtained from HTL under various solid concentrationsand temperature using (DCM).
RT (min) Name of Compound
Area (%)
Slurry Concentration atFixed 350 ◦C Temperature
Temperature at Fixed 25%Slurry Concentration
15% 25% 30% 280 ◦C 300 ◦C 350 ◦C
4.01 2-methyl pyrimidine 3.92 - 4.05 4.94 8.18 -4.41 4-hydroxy-4-methyl-2-pentanone - 5.03 - 23.8 4.15 4.034.82 Ethylbenzene 0.68 8.86 - 1.34 1.75 8.864.97 1,4-dimethyl benzene - 11.9 - 2.48 2.77 11.35.67 2-methyl-2-cyclopenten-1-one 4.71 - 2.72 9.19 10.0 -5.79 2,5-dimethyl pyrazine 4.84 4.09 3.53 12.1 12.4 4.096.8 3-methyl-2-cyclopenten-1-one 6.66 4.86 2.31 2.7 6.0 4.57.1 Phenol 3.24 3.33 2.16 - 2.43 3.328.15 2,3-dimethyl-2-cyclopenten-1-one 12.8 12.0 5.32 2.29 5.73 6.059.03 1-acetate-1,2,3-Propanetriol 4.72 5.09 5.63 2.58 4.22 5.199.27 Undecane 9.20 11.1 12.6 2.49 5.9 8.999.94 1,3-diethyl-3-methyl-2,5-pyrrolidinedione 2.27 2.5 2.72 - 2.48 2.51
10.69 1-propyl-2-pyrrolidinone 3.04 2.32 3.48 - - 2.3011.39 1-butyl-2-pyrrolidinone 3.3 4.25 3.16 - - 4.2024.91 Di(2-propylpentyl) ester, phthalic acid - 10.7 8.25 - - 10.7
-, chemical compounds either not detected or peak area less than 2%.
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As shown in Table 3, cyclic oxygenated compounds (mostly ketones such as 2,3-dimethyl-2-cyclopenten-1-one), are slightly decreasing with increasing solids concentration. This was in contrastto the aliphatic components, such as undecane, which increased with the solid concentration. Cyclicnitrogenated and oxygenated compounds including 1-butyl-2-pyrrolidinone, 1,3-diethyl-3-methyl-2,5-pyrrolidinedione, and 1-propyl-2-pyrrolidinone were the most abundant in each sample.In the low temperature reaction at 280 ◦C, bio-crude oil had the highest amount of oxygenatedcompounds mostly aliphatic, prevalent ketones, including 4-hydroxy-4-methyl-2-pentanone,2-methyl-2-cyclopenten-1-one, and 2,3-dimethyl-2-cyclopenten-1-one. Abundance of hydrocarbonsand aromatics were the lowest for the bio-crude oil at 280 ◦C but had the highest percentages at350 ◦C. The number of identified nitrogenated compounds such as pyrazine, 2-methyl-2,5-dimethyl,pyrimidine, trimethyl and pyrazine, initially, increased with temperature and reached the maximumat 300 ◦C after which it decreased. Jena et al. obtained similar results from bio-crude oil comparingfive different temperatures [23]. Esters such as Di(2-propylpentyl) ester, phthalic acid, 15-methyl ethylester, and heptadecanoic acid, were observed only at the highest temperature. The detailed analysisshows the strong dependency of chemical compounds on process condition.
3.5. Effect of Temperature and Concentration on Chemical and Physical Properties
Both the chemical and physical properties of biofuels are important to define fuel quality in termsof combustibility, density, energy content, and lubricity. These fuel properties vary with chemicalcomposition and influence engine performance and emission results [20,45–49]. It was observedthat the chemical components and compositions of the bio-crude varied with the process conditions(Tables 2 and 3) which subsequently affected the fuel’s physical properties. Table 4 shows the effect oftemperature and concentration on ultimate analysis and higher heating value (HHV) of the microalgaebio-crude. The chemical and physical properties of microalgae HTL bio-crude are shown in Table 5and compared with FAME microalgae biofuel and mineral diesel.
Table 4. Ultimate analysis and higher heating value (HHV) of the microalgae bio-crude.
Component (wt. %)25% Slurry Concentration at
Different Temperature350 ◦C Temperature at Slurry
Concentration
280 ◦C 300 ◦C 350 ◦C 15% 25% 30%
C 68.1 70.4 75.6 74.1 75.6 73.7H 9.3 8.9 10.1 9.7 10.1 9.8O 15.7 12.1 10.3 11.2 10.3 10.8N 6.9 8.6 4.0 5.0 4.0 5.7
H:C 0.14 0.13 0.13 0.13 0.13 0.13O:C 0.23 0.17 0.14 0.15 0.14 0.15N:C 0.10 0.12 0.05 0.07 0.05 0.08
HHV (Cal.), MJ·kg−1 33.5 34.3 37.4 36.4 37.5 36.5HHV(Meas.), MJ·kg−1 – 26.1 29.8 26.5 29.8 28.0
3.5.1. Bio-Crude Chemical Composition
The extracted bio-crude contained a range of complex hydrocarbon groups including aliphatics,aromatics, as well as nitrogenated and oxygenated compounds. Hydrocarbons mostly containedeither oxygen or nitrogen in the C–H chain. Table 4 shows the elemental composition of the bio-crudeobtained at different temperatures with 25% slurry concentrations and for different slurry concentrationat 350 ◦C. The elemental analysis showed that the mass percentage of oxygen decreased from 16%to 10% with increasing temperature of the experiment for 25% slurry. In contrast, the hydrogenpercentage was relatively stable: within 8.93–10.14% over the experimental range. The differencesin chemical composition and molecular structure (e.g., C, H, and O composition, straight chain,cyclic, and heterocyclic compounds) affect the physical properties of the bio-crude oil including
Energies 2017, 10, 467 10 of 16
higher heating value, density, viscosity, cetane number, and surface tension; important parametersfor internal combustion (IC) engines. The slow heat-up and cooling down time can potentiallylead to polymerisation and secondary reactions. It is expected that the engine performance andexhaust emissions would be different compared to FAME biodiesel. For instance, the oxygen createsa permanent dipole moment which results in stronger hydrogen bonding and oxygenated fuelswith increased molecular affinity. Consequently, compressibility is decreased because the free spacebetween the molecules is similar. A follow-on effect is an increase in NOx [45]. Nabi et al. [50] hasalso shown NOx emissions and adiabatic flame temperature present a linear decrease with increasingoxygen content.
O:C, N:C, and H:C significantly affect fuel quality and emissions, and so the temperature variationand slurry concentration effect on bio-crude were compared with diesel and FAME biodiesel standardsin the Van Krevelen diagram (Figure 5) [51,52]. It is important to note that the lowest N:C and O:Cwere found at the same conditions which gave maximum bio-crude yield and HHV (i.e., 350 ◦C and25% slurry concentration; for this range, the author’s viewed lower O:C to be more beneficial). Figure 5shows that O:C changes with respect to N:C ratio, and H:C is almost constant; thus, N:C and O:Cvaried more than H:C. The HHV is the highest at 350 ◦C due to the low O:C and relatively good H:C.N:C also sharply reduced at high temperature. The bio-crude oils were more similar to FAME biodieselthan fossil fuel diesel in terms of H:C while the nitrogen concentration was much higher than dieseland biodiesel standards due to the high amount of proteins in the raw feedstock. We note that ifn-hexane had been used for the extraction rather than DCM the polarity of the bio-crude would falland, consequently, the point would shift to the left on Figure 5 due to the lower oxygen content.
Energies 2017, 10, 467 10 of 15
3.5.1. Bio-Crude Chemical Composition
The extracted bio-crude contained a range of complex hydrocarbon groups including aliphatics, aromatics, as well as nitrogenated and oxygenated compounds. Hydrocarbons mostly contained either oxygen or nitrogen in the C–H chain. Table 4 shows the elemental composition of the bio-crude obtained at different temperatures with 25% slurry concentrations and for different slurry concentration at 350 °C. The elemental analysis showed that the mass percentage of oxygen decreased from 16% to 10% with increasing temperature of the experiment for 25% slurry. In contrast, the hydrogen percentage was relatively stable: within 8.93–10.14% over the experimental range. The differences in chemical composition and molecular structure (e.g., C, H, and O composition, straight chain, cyclic, and heterocyclic compounds) affect the physical properties of the bio-crude oil including higher heating value, density, viscosity, cetane number, and surface tension; important parameters for internal combustion (IC) engines. The slow heat-up and cooling down time can potentially lead to polymerisation and secondary reactions. It is expected that the engine performance and exhaust emissions would be different compared to FAME biodiesel. For instance, the oxygen creates a permanent dipole moment which results in stronger hydrogen bonding and oxygenated fuels with increased molecular affinity. Consequently, compressibility is decreased because the free space between the molecules is similar. A follow-on effect is an increase in NOx [45]. Nabi et al. [50] has also shown NOx emissions and adiabatic flame temperature present a linear decrease with increasing oxygen content.
O:C, N:C, and H:C significantly affect fuel quality and emissions, and so the temperature variation and slurry concentration effect on bio-crude were compared with diesel and FAME biodiesel standards in the Van Krevelen diagram (Figure 5) [51,52]. It is important to note that the lowest N:C and O:C were found at the same conditions which gave maximum bio-crude yield and HHV (i.e., 350 °C and 25% slurry concentration; for this range, the author’s viewed lower O:C to be more beneficial). Figure 5 shows that O:C changes with respect to N:C ratio, and H:C is almost constant; thus, N:C and O:C varied more than H:C. The HHV is the highest at 350 °C due to the low O:C and relatively good H:C. N:C also sharply reduced at high temperature. The bio-crude oils were more similar to FAME biodiesel than fossil fuel diesel in terms of H:C while the nitrogen concentration was much higher than diesel and biodiesel standards due to the high amount of proteins in the raw feedstock. We note that if n-hexane had been used for the extraction rather than DCM the polarity of the bio-crude would fall and, consequently, the point would shift to the left on Figure 5 due to the lower oxygen content.
Figure 5. Van Krevelen diagram of bio-crudes gained for different temperatures (280°C, 300°C, and 350 °C) and slurry concentration (15%, 20% and 30%) in comparison with diesel and FAME biodiesel standards [53].
Figure 5. Van Krevelen diagram of bio-crudes gained for different temperatures (280 ◦C, 300 ◦C, and350 ◦C) and slurry concentration (15%, 20% and 30%) in comparison with diesel and FAME biodieselstandards [53].
The separated bio-crude oil samples had a dark colour, high viscosity, and an acrid smoky odour.Chemical and physical properties were analysed for the best bio-crude produced at 350 ◦C, and 25%initial solids concentration and compared with various types of fuel standards (Table 5).
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Table 5. Comparison of chemical and physical properties of bio-crude produced at 350 ◦C and25% solids with trans-esterified microalgae biodiesel, diesel, biodiesel, and marine fuels standards.*: at 20◦ C.
Name of the Properties HTL Scenedesmussp. Biocrude Oil
FAMECrypthecodinium
cohnii Biodiesel [52]
BiodieselStandards EN
14214
PetroleumDiesel
MarineFuels ISO
8217
Kinematic viscosity@40 ◦C (mm2·s−1) 70.7–73.8 5.06 3.5–5 2.64 1.4–11.0Density @15 ◦C (kg·L−1) 0.97 * 0.91 0.86–0.9 0.84 0.96–0.99
HHV (MJ·kg−1) 29.7 39.8 - 44 44–45Oxygen content (wt. %) 10.3 10.4 - 0 -
Hydrogen Content (wt. %) 10.1 11.1 - 13.8 -Carbon Content (wt. %) 75.5 78.4 - 86.1 -
Nitrogen Content (wt. %) 3.97 0 - 0 -
3.5.2. Viscosity
Under the best conditions, the viscosity of the HTL microalgae bio-fuel was closer to conventionaldiesel and biodiesel than FAME from microalgae and varied with chemical composition, possiblydue to the varying degree of chemical saturation [54]. The variation of bio-crude compositionsinfluenced the intermolecular forces. The variation in viscosity can potentially affect injection timing,spray, atomization, and combustion compared to microalgae FAME [28]. There is a possibility to usebio-crude in a heavy-duty diesel engine or a marine ship engine with minimal upgrading.
3.5.3. Density
The fuel density affects the mass of fuel injected because fuel injection systems in modern dieselengines measure the fuel on a volume basis [45]. At best conditions, the density of the HTL microalgaebio-crude was 13% higher than conventional diesel fuel and not comparable with any fuel standardsin Table 5. The increased density of the HTL microalgae bio-crude might be due to it containing manyaromatic hydrocarbons and cyclic chemicals as well as an amount of high molecular weight compoundsthat are beyond detection in GC-MS. However, the density value (0.97 kg·L−1) is comparable to themarine residual fuel standard ISO 8217:2012 (0.96–0.99 kg·m−3).
3.5.4. Higher Heating Value
Among the fuels, the HHV of microalgae bio-crude had the lowest value 29.8 MJ·kg−1
(best H:C) [55]. Enrichment in double bonds generally results in lower than expected levels ofheats of combustion due to strong intra-molecular bonding [56]. The vapour heat capacity and thermalconductivity relates to the heating value of the fuel. These affect droplet-surrounding heat transfer,temperature distribution and the mass air fuel ratio that will reduce combustion performance [57].
3.6. Comparison with Previous Studies
The findings of this study are compared with previous workers in Table 6 where most researchersworked with very small batch reactors (<100 mL) although there has been a few studies with largercontinuous reactors (typically ~1000 mL). The vast majority of HTL studies report physical properties,HHV in particular, derived from correlations which use elemental analysis results for gaseous, liquid,coal and biomass materials [36,39]. Our calculated values for HHV are similar to that reportedelsewhere, but we also report a much lower measured value which was also reported by Li [58],although despite the different oxygen content.
The density and viscosity of the bio-crude are not widely reported (Table 6). The bio-crudeviscosity was significantly closer to regular diesel and biodiesel standards than transesterified (FAME)microalgae biodiesel. Under optimised conditions, HTL bio-crude’s high density (0.97–1.04 kg·L−1)and its high viscosity (70.7–73.8 mm2·s−1) had enough similarity to marine heavy fuels that it could beimmediately used without further processing, although the measured higher heating value, HHV, waslower (29.8 MJ·kg−1).
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Table 6. Comparison of results from this study with literature.
HTL Bio-Crude Reactor TypeReactorVolume
(mL)
Operating ConditionsYieldwt. %
Physicochemical Properties
ReferencesTemperature(◦C)
Time(min)
Chemical Composition % HHV (MJ/kg) KinematicViscosity (mm2/s)
Density(kg/L)C H N O Calculated Measured
Scenedesmus sp.
Batch reactor
1800 350 60 33.6 75.6 10.1 3.97 10.3 37.4 29.8 70.7–73.8 0.97 Current study500 300 30 45 72.6 9.0 6.5 10.5 35.5 - - - [59]
Enteromorpha prolifera 25 300 30 23 64.5 7.7 5.4 22.4 30.8 - - - [60]Lemna sp. 25 350 30 17.5 72.1 7.8 4.6 15.5 32.8 - - - [61]
Laminaria saccharina 75 350 15 19.3 82.0 7.1 4.9 6.0 37.4 - - - [27]Chlorella pyrenoidosa 17.2 350 60 41 75.1 9.9 7.3 7.7 38.1 - - - [62]Nannochloropsis sp. 35 350 60 43 76.0 10.3 3.9 9.0 39 - - - [32]
Enteromorpha prolifera 25 370 40 31.7 77.9 9.6 5.6 6.9 39.4 - - - [10]Sargassum patens C. Agardh 1000 340 10 32.1 64.6 7.4 2.5 25.5 - 27.1 - - [58]
Saccharina sp. Continuous-flowreactor
1000 340 87 58.8 79.4 8.0 4.1 8.5 37.5 - - - [63]NB238 1000 350 - 38 78.6 10.4 4.2 5.3 - - - - [64]
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4. Conclusions
This paper studied the relationship of HTL operating conditions (reaction temperature and slurryconcentration) on the chemical pomposities and physical and chemical properties of the microalgaebio-crude. Physical properties were able to be measured experimentally because of the larger scaleof the reactor rather than being derived using correlations based on elemental analysis. We notea significant difference in the values for calculated and measured values, which is consistent withthe literature. The highest bio-crude oil yield (33.6%) was produced at 350 ◦C and at 25% solidsconcentration. The aliphatic, aromatic, nitrogen containing hetero-cyclic, oxygenated compounds werethe major group of components of HTL microalgae bio-crude. These conditions also produced thelowest O:C and N:C. The bio-crude had higher density and lower HHV than diesel and biodiesel andwas closer in character to heavy fuel oil where it could be used directly. Future work will focus onfurther improving chemical and physical properties, such as HHV, density, and decreasing N and Opercentages, via catalytic upgrading, or via reactions in situ, followed by engine testing.
Acknowledgments: This work was financially supported by the QUT Early Career Academic Recruitment andDevelopment program and by a Ph.D. scholarship from the QUT School of Chemistry, Physics, and MechanicalEngineering. The authors thank to the QUT Central Analytical Research Facility for their assistance.
Author Contributions: All authors contributed to this publication and have read and approved the final manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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